1. Technical Field
The present invention relates to a transistor, and more particularly, to a metal oxide semiconductor field effect transistor (MOSFET) and a method of fabricating the same.
2. Description of the Related Art
To achieve a higher integration of semiconductor devices, the size of the semiconductor devices needs to be reduced. Particularly, a critical dimension (CD), which generally equals the length of a gate electrode of a metal oxide semiconductor field effect transistor (MOSFET), needs to be reduced. However, a reduced CD results in a reduced channel length of a MOSFET, which causes various problems often referred to as short-channel effects.
Short channel effects deteriorate the characteristics of the MOSFET. For instance, as the channel length decreases, a threshold voltage decreases, a drain induced barrier lowering (DIBL) phenomenon occurs, and a leakage current between a drain and a source increases.
In order to prevent or alleviate the short channel effects, there is proposed a method for implanting impurities for threshold voltage adjustment over an entire surface of a channel. However, this method does not allow high integration of semiconductor devices.
Alternative methods for alleviating the short channel effects have also been proposed. For example, in order to adjust a vertical doping profile, a super steep retrograde (SSR) well or a pocket implantation may be formed. For another example, halo ion implantation regions formed in lower portions of the source/drain regions may also be formed to reduce the short channel effects.
FIG. 1 is a cross-sectional view of a semiconductor device illustrating a related art method for forming halo ion implantation regions.
Referring to FIG. 1, a gate oxide 7 and a gate electrode 9 are formed on a semiconductor substrate 1. Semiconductor substrate 1 may comprise silicon. Semiconductor substrate 1 includes a low concentration impurity region 3 and a high concentration impurity region 5 acting as a source or drain region on each side of gate oxide 7 and gate electrode 9. Oxide spacers 11 are formed on sidewalls of gate electrode 9 and gate oxide 7.
To form halo ion implantation regions 13, impurities are implanted into lower portions of the source/drain regions using gate electrode 8 and oxide spacers 11 as an ion implantation mask. Halo ion implantation regions 13 prevent the reduction in the threshold voltage and preserve the channel mobility.
However, a problem with the related art halo ion implantation method is that a junction capacitance may be increased and a junction depth is affected because the halo ion implantation regions are formed in source/drain regions as well as the semiconductor substrate region. Accordingly, when integration density is high, the threshold voltage of device with halo ion implantation regions is difficult to control.